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The crustal structure of the marmara region using receiver function analysis
The Marmara Region is a rapidly deforming area with high seismic activity in the northwestern Turkey. In order to further understand the crustal structure in the region, we present results from receiver function analysis using the permanent stations in the region by applying H-κ stacking algorithm which gives crustal thickness and Vp/Vs ratio beneath a station. 40 land stations between January of 2008 and April of 2012, and five cabled Sea Bottom Observatories which were deployed at the end of 2010 by Kandilli Observatory and Earthquake Research Institue located between 40.2°-41.2° N and 26.5°-30.5° E were included in the analysis. Approximately 250 teleseismic events from a wide range of epicentral distances with magnitudes greater than Mw 5.5 are used to obtain receiver functions. Furthermore, in order to calculate the receiver functions in time domain using iterative deconvolution technique suggested by Ligorria and Ammon (1999). Consequently, the crustal structure of the region has been reasonably defined and compared with the other studies. As a consequence of the receiver function analysis, the Moho depth variation map and Vp/Vs ratio map were plotted. The Moho depth on average is 31 km. There are no sharp changes in the crustal thickness of the Marmara Region except North Marmara Trough because basin structure of the Marmara Sea where crustal thickness reaches up to 26 km in the same region was not observed. Furthermore, we found overall average Vp/Vs ratio of 1.74, for the region but we obtained low Vp/Vs ratios in the stations located near Çınarcık Basin which varies between 1.64 - 1.74 indicating the effect of basin structure in the area and North Marmara where Vp/Vs ratios vary between 1.60 and 1.70 whichis related to the sediment structure of the area. We also acquired higher Vp/Vs ratios which are between 1.86 and 1.96 in the Western Marmara Region. This can be due to increasing mafic content in this area. Additionally, an attempt has been made to invert the radial receiver function of the station KCTX using iterative linear 1D inversion method in order to compare the Moho depth values with two different techniques.
A rapid estimation of moment magnitude mw for the October 23, 2011 Van earthquake using strong-motion records
It is well known that the Richter scale (local magnitude scale, ML) based on the measurement of the amplitude in a narrow- band time domain saturates for large crustal earthquakes, and the possibility of observing clipping on seismograms is high. Therefore it is insufficient to use the local magnitude for reliable magnitude estimation. A typical example of this situation is confronted in case of the October 23, 2011 Van earthquake ( ML 6.6 vs Mw 7.2) . Here Mw is the moment magnitude obtained from the far-field displacement spectrum of body waves. On the other hand, a better match is expected in the range of 3 < ML < 7 because in this range the empirical relation between seismic moment M0 and ML is almost the same with the relation between M0 and Mw (Thatcher and Hanks, 1973; Hanks and Kanamori, 1979). Hence, there is a need to find a way for the determination of Mw from near-field records too.The aim of this study is to determine moment magnitude of earthquakes very soon after occurrence, before any source or focal mechanism inversions have been performed. In this respect (Delouis et al.,2009) proposed the MWSYNTH method. In this method, observed displacement spectra are compared with synthetic spectra computed for a variety of finite dimension source models scaled with Mw. In this study we have tried to obtain a rapid estimation of Mw for the October 23, 2011 Van earthquake from the strong-motion records in the epicentral distances about 120km using MWSYNTH method. Using strong motion records of only two stations we obtained an agreeable estimation of moment magnitude Mw=6.9. The result may be improved (approach more seriously to the reference moment magnitude Mw ref=7.15) by doing computation for more stations. And it is foreseen that a correct recovery of Mw for the Van Earthquake within about 100 sec after origin time (recording+computing time) will be possible if the procedure is automated.
Accurate location of hypocenters using double difference and active fault structures in Gökova Bay
Double Difference Algorithm, HYPODD, is used for relocating the earthquakes in the Gulf of Gökova. The aim of this thesis is two folds: first we look for the best choice of inversion parameters which determine the performance of HYPODD at local scale, and second, as a by product of the test data used in the study, we determine the active faults in the Gulf of Gökova. We used four year (April 2006-December 2009) seismic earthquake recordings and relocated 972 events with magnitudes between 1.5 and 4.5. The inversion part of HYPODD package can be run by using both catalog and cross- correlation data. In this study both methods were used. We have observed that correlation based inversion gives a better picture only if the events in the cluster are very close to each other (<3km). When stations are sufficiently high in number (>4 stations) and well scattered around the seismic zone at moderate distances (i.e. <60 km), we observed that the performance is high, and do not critically depends on the control parameters. The improvement in using hypoDD and in particular correlation based applications is mostly apparent when depth sections are analyzed. The other important observation is that the choice parameters and therefore the final performance entirely depend on the geometry and the distance of event pairs. The parameters MAXSEP, MINLNK, MINOBS are very critical and a conservative selection of these parameters will lead to a drastic reduction of the data set. Separating the data into clusters or not is a matter which entirely depends on the data. If data shows isolated clusters with distinct character each, it would be unrealistic to use a single set of control parameters for all of them, and clustering is recommended. In term of active fault geometry of the faults in Gökova, it is clear that an offshore fault parallel to the northern boundary is well confirmed. The fault extends from midway between Ören and Çökertme to land close to Akyazı, roughly 27 45'W to 28 20'W. The depth section of this fault is vertical in the central part, but shows a possible south dipping in the east. At the western end the fault shows a change in strike and turns south with a strike direction of roughly 36. This fault continues to 36 45'N, midway between Cos Islands and Datça Peninsula.
Modelling 3D seismic wave propagation in Marmara Region
This study focuses on the modeling of 3D seismic wave propagation in the east of the Marmara Sea in particular for the city of Istanbul, which is identified as one of the megacities with the highest seismic risk in the world. For the first time, an attempt is made for creating a 3D seismic model and for testing the new model with real data. In the frame of constructing 3D velocity model, previous crustal studies of Marmara region and all other available field data, including surface and borehole measurements, are compiled to form a collection of 1D models. Each 1D model relates to a specific location point inside the study area. We have used interpolation methods, in particular Delaunay triangles approach, in order to fill in the no-data zones, which separate the 1D observation points. Elastic wave propagation is simulated inside the newly created 3D model using finite difference approach. An open source code called Wave Propagation Program (WPP), which operates on parallel processing environment, is used for that purpose. We have tested the performance of the 3D model with real data using the earthquake of September 29, 2004 (Ml=4.1) occurred in Çınarcık Basin, which was recorded by 18 permanent broadband stations and 100 strong motion stations. A detailed analysis of the source properties of the event is done, both for the location and the fault plane solution. Real and synthetic waveforms are compared both in time and frequency domains. Matching of the waveform shapes are studied in detail. In each case improvement of 3D model over 1D counterpart is discussed. A more quantitative evaluation of 1D and 3D performances is carried out using waveform correlation. The final result shows that a considerable improvement is achieved with 3D model both in terms of amplitudes and P and S arrival times. The finite difference method is also applied to specified basin structures filled with soft sediments of low shear velocities. Sabiha Gökçen Airport area in Pendik, is studied in detail because its basement geometry and sedimentary cover are well-known. The analysis, performed both in the time and frequency domain, helps to understand the characteristics of the 3D wave propagation inside the basin and the site effects related to it.
Analysis of the seismic activity in Tuz Gölü Fault Zone and surrounding areas following the 2023 East Anatolian earthquakes
The aim of this study is to examine whether the major earthquakes that occurred in the East Anatolia in 2023 had an impact on the Tuz Gölü Fault Zone and its surroundings and to carry out earthquake analysis of the region since the earthquakes. During the study, the seismic activity of the region in the 9-month period starting from the February 6 earthquakes was examined using Kandilli Observatory and AFAD data. Focal mechanism solutions and relocation studies were also carried out to obtain detailed results. Following the 2023 Eastern Anatolia earthquakes, significant seismic activity was detected in the south of the Tuz Gölü Fault Zone in Central Anatolia. In this study, earthquake relocations and focal mechanisms related to this activity are discussed. The relocation and focal mechanism solutions of the seismic activity of the region in the two-month period starting from the February 6 earthquakes shows that the activity occurred on two almost N-S structures that are to the west of Hasandağı Volcano. The mechanisms and epicenter locations are consistent with a right-lateral N-S fault for the eastern activity with some normal component. The activity might be related to the volcanism considering that they are on the proximity of fissures to the west of Hasandağı.
Solving ray-tracing equations with neural networks
This thesis presents an innovative approach for solving ray-tracing equations using neural networks, offering faster and more accurate solutions. The primary objective is to solve ray-tracing equations with neural networks, however, we decided to initially examine the capabilities of the network in addressing a one-dimensional problem, such as the harmonic oscillator. Firstly, we compare various neural network models for solving the harmonic oscillator equation. These include conventional neural networks, physics-informed neural networks (PINNs), and a combined model the Hamiltonian approach with the PINN. Conventional neural networks require balanced data to solve the problem and struggle when data is not provided. Conversely, PINNs are able to learn successfully with a small dataset by satisfying physical equations, for both provided and unprovided data regions. However, the aim is to solve these equations with only initial conditions, without providing any additional data. To achieve this, a neural network model integrating the Hamiltonian approach with PINN is introduced. This integrated model mainly focuses on energy conservation, enabling the model to find solutions using initial conditions. The application of this thesis is solving ray-tracing equations in linearly heterogeneous media, both isotropic and anisotropic using the integrated model. We solve ray-tracing equations for an isotropic continuum in the forward problem by providing parameters like velocity, take-off angle, and time. The model efficiently determines seismic wave ray paths. In the inverse problem, we aim to determine the angle and velocity model for an isotropic continuum by providing receiver and source locations, and travel time. We then progressively increase the complexity of the problem, starting with isotropic velocity model and advancing to constant elliptically and to generalized elliptically anisotropic models.
Bayesian modeling of co-seismic slip using GNSS data: 2002 Çay earthquake
In southwestern Turkey, in the Afyon-Sultandağı region, a mainshock of magnitude Mw 6.5 occurred at 07:11:28 (GMT) and was followed approximately two hours later at 09:26:49 (GMT) by an aftershock of magnitude Mw 5.8. The mainshock took place south of Eber Lake, causing surface discontinuities and significant surface deformation. Numerous researchers have studied the earthquake's source parameters using traditional teleseismic and geodetic data. However, to better visualize the co-seismic deformation and more accurately determine the geometry and slip distribution of the seismogenic fault, a Bayesian approach was adopted in this study. The Bayesian approach creates a posterior probability density function (PDF) that encompasses all possible values of model parameters by incorporating observations, prior physical knowledge of the source model, and the distribution of differences between forward model predictions and observations. In this study, geodetic datasets were utilized for both the rectangular source model and finite fault modeling. The results confirm that the 2002 Çay Earthquake predominantly involved a normal faulting mechanism with a significant left-lateral strike-slip component. The fault geometry, characterized by a strike angle of 277°, a dip angle of 43°, and a rake angle of -56°, aligns well with the regional tectonic framework. The uniform slip model, validated by the Bayesian method, effectively captured surface deformation patterns and achieved a variance reduction of 92%. This finding highlights the power of Bayesian methods in accurately modeling fault dynamics, particularly in regions with limited seismic data.
Determination of upper mantle heterogeneity beneath Aegean-Anatolian region from travel time tomography
The objective of this work is to determine the heterogeneities of the upper mantle in the Aegean-Anatolian domain using teleseismic tomography. A waveform dataset was prepared from 798 teleseismic earthquakes with magnitudes greater than 5.5 between January 2004 and December 2015. 417 stations from permanent and temporary networks with more than 64,000 direct P phases are used in the computations. The relative travel times of P waves with respect to the ak135 (Kennett et al. 1995) earth model are computed using waveform cross-correlations technique. The tomographic images are computed as perturbations with respect to ak135 earth model. An algorithm named as fast marching method (FMM) (Sethian, 1996a, 1996b) based on the solution of Eikonal equation is used in the forward computation of the travel times. The inversion is performed using subspace inversion scheme. Trade-off curves are plotted and several synthetic tests are performed in order to select optimum parameters (damping and smoothing) for tomography and the resolution and model roughness were investigated. The tomographic images obtained to a depth of 700 km. The computed tomographic images show a heterogeneous upper mantle structure in the Aegean-Anatolian domain. The results are similar to the previously published images mostly but provides higher resolution for the study area. Both Hellenic and Cyprus subductions are imaged to the depth of 700 km. The tear (Pliny-Strabo Tear) between two subduction zones is clearly observed reaching to 660 km discontinuity. A smaller scale tear (Antalya Bay Tear) is also observed on the Cyprus slab around Paphos Transform Fault. The Anatolian plate is underlined by low velocity mantle material with thickness increasing from west to east. The northern block of the North Anatolian Fault (NAF) is observed as high velocity body observable to a depth of 100-200 km. NAF has a sharp velocity contrast between the north and south.
Lithospheric structure of the Western Turkey and Aegean region
Aegean-Anatolia region undergoes an intense internal deformation as evidenced by the existence of major active faults, intense seismic activity and the marked thinning of the crust. It makes the region center of attraction to the study the interaction between the deep structure with the surface deformation. The aim of this study is to provide constraints on the crustal and uppermost mantle structure by using seismic data of permanent broad-band network of Kandilli Observatory and Earthquake Research Institute (KOERI-RETMC), and a temporary array of Seismic Imaging beneath Aegean-Anatolia Domain (SIMBAAD) experiment. Seismic stations of Republic of Turkey Prime Ministry Disaster and Emergency Management Presidency (AFAD), Incorporated Research Institutions for Seismology (IRIS) and previous experiment called Western Anatolia Seismic Recording Experiment (WASRE) were used to complement the network. In this regard we present two high resolution lithospheric images along a ~650 km transect crossing western Anatolia at 28°E longitude from the Black Sea to the Mediterranean and a ~550 km transect crossing central Anatolia at 30.5°E longitude. A total of 5250 receiver functions are computed from the records of teleseismic earthquakes at 40 broadband seismic stations for each of the profiles with an average spacing of ~ 15 km. Lateral variations of crustal thickness, Vp/Vs are inferred from both H-K, and common conversion point stacks (CCP). In order to have a better idea on the accuracy of the estimated crustal parameters we also performed a search scheme based on the Neigboorhood Algorithm. The receiver functions are inverted for a 1-D layered medium to determine the layer thicknesses, Vs and Vp/Vs. The CCP images reveals a long-wavelength variations of Moho depth from ~31 km in the Thrace basin to ~25 km beneath the Marmara Sea, ~25 km beneath the Menderes Massif and ~20 km on the coast of the Mediterranean on the western Anatolia transcent. On the eastern transect, a smooth Moho topography is observed with a sharp discontinuity at depths ranging from 34 km beneath the Black Sea coast, ~35 km beneath the Sakarya Zone with mafic composition to 43 km beneath the Antalya Bay on the central Anatolia profile. The Moho of the subducted African lithosphere is imprinted between ~40 and ~60 km depth at the southern end of the western Anatolia profile, dipping northward where the subducted Cyprus lithosphere is observed dipping northward with an angle of 40◦ between ~40 and ~100 km depths beneath the Antalya Bay on the central Anatolia transect.
Marmara Denizi'nde dinamik deprem kirilmasi simülasyonlari
The 1912 Mürefte and 1999 Izmit M_w7.4 earthquakes are the last devastating events of the western and eastern sections of the Marmara region, respectively. The center of the Sea of Marmara, the region between locations of these two earthquakes, is prone to creating another large earthquake. The main objective of our study is to determine 3D dynamic earthquake rupture scenarios, considering non-planar and heterogeneous stress distribution in the Sea of Marmara. Recent studies show that some segments of the North Anatolian Fault (NAF) beneath Marmara are partially creeping. In this study, it is the first time that we attempt to generate realistic earthquake scenarios by putting constrains on initial stress on the fault using regional stress from earthquake focal mechanisms, in addition to stress release during past earthquakes and strain accumulation during interseismic period using geodetical measurements on slip-rate and locking depth at various segments along the NAF beneath the Sea of Marmara. In order to constrain the regional stress in addition to our previous five cluster analysis a new earthquake cluster is analyzed in the Central Marmara Basin. We use 3D Finite Element Method (PyLith) for dynamic earthquake simulations and tetragonal mesh for better smoothing at the fault bends, which allows us to implement nonplanar fault geometry and initial stress heterogeneity using slip-weakening friction law. We place constraints on initial shear stress from geodetic and seismic studies of locking depth and interseismic strain accumulation. We consider 80 rupture scenarios and calculate slip distribution, rupture velocity and moment magnitude in addition to slip-rate and traction on the fault surface, and displacement and velocity on the ground surface. We find that for the most scenarios possible earthquake magnitude does not exceed Mw7.2. In addition, in none of the possible scenarios we obtain super-shear rupture velocity. We find that depending on the location of the initiation point, asperities in the partially creeping segments and loaded initial stress, the rupture may not extend into the Prince's Island Segment.
Studying seismotectonics of Eastern and Southern Anatolia using earthquake mechanisms
The Anatolia-Aegean domain provides a unique opportunity to explore plate interactions where oceanic subduction, continental collision and transform plate motions are observed simultaneously. High seismicity rates and diversity of the earthquake source mechanisms are the result of the accommodation of these relative plate motions. As the initial tectonic buildup involves the amalgamation of different tectonic units, it is natural that lithospheric segments with varying structural properties in this relatively small region also contributes to the complexities of the observations. Understanding interactions of these plates and related deformation requires an integrated analysis of various observations such as seismic tomography, earthquake slip models, geodetic observations and stress changes along with the seismicity and earthquake source mechanisms. In this thesis, 3 case studies in different tectonic settings are presented: the continental collision in the east, the extension due to roll back in the west and the transition between extension and compression. For these 3 case studies, the relation of earthquake source mechanisms to other seismological and geodetic data is used to better understand the present state of the seismotectonics of Easternmost Mediterranean including eastern Anatolia. The October 23, 2011 Mw7.1 Van, Eastern Anatolia earthquake which is on an EW trending thrust fault, in a region under N-S compression due to the convergence of the Arabian plate toward Eurasia. The three faults were activated during and after the coseismic rupture. The earthquake source mechanisms with consistent orientations are grouped in three clusters. An average fault mechanism is calculated for each cluster by the summation of moment tensors. The triggered faults have experienced Coulomb stress increase due to co-seismic rupture revealing a mechanism which accommodates NS shortening in the region. The June 20, 2017 Mw 6.6 Bodrum-Kos earthquake which occurred on an E-W trending normal fault is related to the roll back effect of Hellenic Subduction. The Bodrum-Kos event revealed that the extension in the western section of Gökova Bay is accommodated by a north dipping fault. Two different fault slip models, dipping to north and south, are used to compute the Coulomb stress changes at different depths. The coherency between the seismicity and the regions of increased stress is used to put a constraint on the dip of the ruptured fault. The gradual change of strikes of aftershock mechanisms from east to west is consistent with the rotation of the strain field region indicating that the observed earthquake pattern during the 2017 earthquake reflects the long term tectonic frame work in the region. In between these compressive tectonics of Eastern Anatolia and extension in the Aegean, Cyprus Arc region acts as a transitional zone which is tectonically less understood. Specifically how the convergence of Nubia toward Anatolia is accommodated remains unclear. By the analyses of novel earthquake source mechanisms, and other seismological and geodetic data, it is proposed that the segmentation of the subducting Nubian Plate has a significant contribution to the lithospheric deformation. The change in the orientations of the earthquake mechanisms around the Isparta Angle determines the eastern boundary of the N-S extension due to roll back of the Hellenic slab and is consistent with the counter clockwise rotation of AnatoliaAegean domain which is revealed by the recent GPS vector field. Thrust mechanism earthquakes along with Bouguer gravity, seismicity, and horizontal GPS velocities reveal the geometry of the subducting slab beneath Antalya Basin towards N-E. We suggest that the Antalya Slab deforms as an isolated block, responding in part to adjacent plates, including the Anatolian Plate that moves toward the west, overriding the remnant Antalya slab.
Interseismic behavior along the North Anatolian Fault in the Marmara Region using 3D structure
A series of earthquakes occurred along the North Anatolian Fault (NAF) during the 20th century, primarily migrating from east to west. The only part of the NAF that has not broken is under the Marmara Sea. The Main Marmara Fault (MMF), the NAF's northern branch, is the most active one, with the highest slip rate amongst the several branches of the NAF. Since the seismic gap of ~150 km is beneath the sea, the geodetic data is not sufficient to constrain the full fault coupling, particularly in the Central Marmara. Nevertheless, the current data does imply that the GNSS vectors along the northern coast of the Marmara Sea are smaller than expected. One interpretation is that the MMF has heterogeneous interseismic coupling with creeping and locked segments. Another explanation is that the fault is locked, but the strain is asymmetrically localized around the MMF as a result of the deep basins. In this study, the competing effects of weak interseismic locking of the MMF and deep basins around the fault are studied by developing a 3-D finite element model for the Marmara Region, which includes a realistic topography, the 3-D geometry of the main fault, and basins, and using the geodetic data as a constraint. Our findings show that the deep basins confine the interseismic strain in the fault vicinity, and using a homogeneous half-space model leads to a slight underestimation of the locking depth. Our 3-D model shows that while the basins have some effects on strain localization, the heterogeneity of interseismic coupling is necessary to explain the observed GNSS data. We infer a change in the locking depth at the Ganos Bend between the strongly coupled Ganos and the weakly coupled Western Marmara. Seismic studies also indicate that these two segments vary considerably in background seismicity. The 50 km creeping segment coincides well with repeating earthquakes and higher rates of diffuse seismicity. Variations in regional stresses and earthquake focal mechanisms, including the 2019 Silivri earthquake sequence, are compatible with the dilatational quadrants in the region due to the loading caused by the interseismic creep of the Western Marmara.
Simplektik geometri ve hamiltonian Monte Carlo metodu
Hamiltonian Monte Carlo (HMC) method is an application of a non-Euclidean geometry to an inverse problem. HMC is a probabilistic sampling method with the basis of Hamiltonian dynamics. One of the main advantages of HMC algorithm is to draw independent samples from the model space with a higher acceptance rate than other Markov Chain Monte Carlo (MCMC) methods. In order to understand how higher acceptance rate is achieved, I have studied HMC in the light of symplectic geometry. Hamiltonian dynamics is defined on the phase space (cotangent bundle), which has a natural symplectic structure, i.e. a differential two-form which is non-degenerate and closed. Hamiltonian function is defined on the phase space, which corresponds to the sum of misfit and the square of the generalized momentum. By using the non-degeneracy property of symplectic form, a vector field can be found in which Hamiltonian function is invariant along the integral curves of the vector field. The invariance of the Hamiltonian function results in high acceptance rate, where we apply accept-reject test to satisfy detailed-balance property. In this thesis, we define some basic concepts and theorems in symplectic geometry, then describe the relation between symplectic geometry and HMC, namely Hamiltonian dynamics. Lastly, we show an implementation for HMC algorithm to a 2D-tomography problem and analyze the tune parameters for application of HMC.
Seismic interactions and related faulting revealed by moderate to large earthquakes in the se aegean region
Southern Aegean and Western Turkey are mainly dominated by significant extensional regime and crustal thinning. Segmented normal faults and horst graben systems with varying orientations characterize the region. In these types of extensional regions, seismicity is quite diffuse and fault networks can be quite complex. Thus, it is challenging to obtain the exact distribution of active faults and understand how they interact with each other to accommodate the extension. In order to interpret the fault structures of the region and their interaction, we focused on Gökova and Kuşadası Bay earthquakes due to the existence of recent high-rate seismicity. For the fault interaction interpretations, we used multidisciplinary data set based fault geometry and fault slip models, source mechanism solutions, InSAR time series analysis. We analyzed the 2017, Mw 6.6 Bodrum − Kos earthquake aftershock progression by using InSAR time series and 3 moderate sized earthquakes that occurred in Ula, which is located on-land in the east of Gökova Bay. Our goal is to reveal the active fault structures on eastern edge of the Gökova Bay, Ula region, by InSAR data modeling of the events, regional seismic waveform inversions and last ten years Mw ≥ 4 magnitude waveform based focal mechanism solutions. As part of this study, 2020 Mw 7.0 Samos earthquake and its aftershocks were also studied. We also applied cluster analysis to identify the changes in waveforms and obtained focal mechanism solutions for most clusters.
High resolution microseismicity and nearly-repeating events in the Marmara Sea
The Main Marmara Fault beneath the Marmara Sea has a prominent seismic gap that can produce a devastating earthquake and a serious risk for the surroundings. It is important to scrutinize the seismic activity in region and relate this activity to the deformation of the fault zone. In this study, a new micro-earthquake database is created for the Marmara Sea between 2014-2016 using the data mostly from ocean-bottom seismometers. The detected and located seismicity indicate that Tekirdağ Basin hosts a diffuse activity from ~7 km to about 18 km depth. A high micro-earthquake activity rate predominates beneath the Central Basin, at depths from 3 km to 15 km. The abundancy of earthquakes in the area can be attributed to a creeping zone, considering the conformity with the geodetic observations. On the other hand, Kumburgaz and the western part of Çınarcık Basins show sparse seismicity at depth ranges of 5-19 km and 3-18 km, respectively, signing to a locked fault compatible with the geodetic observations. In addition to micro-seismicity, the repeating events are detected using template matching method on the continuous waveforms from 2008-2021. The clusters of highly correlated detected earthquakes, which are closely spaced or partially overlapped, are attributed to the "near-repeating earthquakes". The nine nearly-repeating earthquake clusters beneath the Central Basin are observed at 8-13 km depths, suggesting seismic creep behavior together with a high seismicity rate. The fault mechanisms of the near-repeater clusters have strike-slip mechanism consistent with Main Marmara Fault zone. The nearly-repeating events have two different patterns of repeating intervals, as long-term and short-term type events. The amount of slip rates from the near-repeater clusters shows varying slip rates but comparable to geodetic rate. The number of near- repeating events decreased significantly after the 2018 and no repeating event is observed during 2019 which Mw 5.8 Silivri earthquake occurred.
Electrical resistivity structure beneath mt. Erciyes Central Anatolia, Turkey
The Central Anatolia (CA) is an intermediate belt between the Pontides and the Anatolide-Tauride blocks. Numerous basins, folds, faults, volcanic fields, and rich mineral resources in the region have attracted the attention of geoscientists for years. Mt. Erciyes is a Quaternary-aged stratovolcano located in CA, which has not been activated for a long time. In this thesis, I aimed to perform three-dimensional (3 - D) imaging of its electrical properties using magnetotellurics (MT) and interpret its structure. MT method has been extensively used in the exploration of volcanic areas. This study can be considered as the first systematic application of the MT performed in the vicinity of Mt. Erciyes. Between 2013 and 2018, deploying 48 wide-band MT soundings forming a grid surrounding the volcano and its location as part of the Continental Dynamics/Central Anatolian Tectonics project (CD/CAT). The obtained data set was used for determining the dimensionality of the survey area, and the realistic electrical structure of the region was achieved with 3 - D solution algorithms including 38 station data. The results showed that the regional geoelectric strike direction is ∼ N25◦E which coincides with the structural strike of the region. For determining the dimensionality of the MT data, phase tensor analyses were performed. The observed wide-band data were inverted using a 3 - D inversion algorithm known as the ModEM © in two distinct ways; with and without topography information to reveal the electrical structure. The 3 - D inversion results revealed three distinct conductivity features: a widespread conductive anomaly found beneath the volcano and two conductive areas in the vicinity of the volcano. When the final model is examined, it is possible to observe that the conductive regions are separated from each other. If the final model is examined together with the geological data, it is noticed that the boundaries of the conductor coincide with the faulting in the region.
Stress drop analysis of earthquakes in the vicinity of ayvacik geothermal reservoir
An earthquake swarm occurred in Çanakkale-Ayvacık, where started with an earthquake of M=5.4 magnitude on February 6, 2017, and more than 4000 earthquakes were recorded in 2 months. A significant number of these earthquakes were larger than $M_w=4$. Most of the activity occurred on the southwest-dipping Tuzla Fault, which is a normal fault with WNW strike. Given that the fault region is a geothermal reservoir where active geothermal power plants operate, it is important to examine the characteristics of these earthquakes more closely. The activity started near a geothermal power plant and expanded along the Tuzla fault. In this thesis, the stress drops of earthquakes that occurred in the Ayvacık Region during the 2017 earthquake swarm was studied. Stress drop is a crucial macroscopic parameter for earthquakes and in geothermal regions, and variations in stress drop might indicate changes in stress conditions, especially pore pressure. The corner frequencies of 106 earthquakes with magnitudes greater than $M_L \geq 3.0$ were calculated from the P waveforms using 2 stations in the vicinity of the activity. We also calculated the moment magnitudes of 139 earthquakes with local magnitudes greater than 2.8 using P wave spectra. Our analysis shows that the stress drop of the earthquakes before and during the 2017 activity is quite variable. In addition, in comparison to the earthquakes that occurred before ($\sim 3 $ MPa), the average stress drop is higher with mean values of about $7-9$ MPa. We infer that the higher average stress drop during the 2017 activity might be related to a change in the pore pressure, where an increase in pore pressure might have led to a decrease in effective normal stress leading to higher slip values and therefore higher stress drops.
Inversion for elasticity tensor of focal region using machine learning algorithms
The moment tensor is an essential tool in seismology to examine the structure of seismic sources. The deformation at the focal region, using combinations of force couples arranged in a 3 x 3 matrix, is represented by the moment tensor. The moment tensor can be expressed as a linear combination of the eigenvectors of the anisotropic focal region's elasticity tensor. The eigenvalues of a vertically transversely isotropic (VTI) elasticity tensor from an occurring moment tensor of a focal region can be obtained, and the precision of this determination depends on the degree of anisotropy. Machine learning optimization involves iteratively enhancing a machine learning model's precision by reducing the error level. Choosing an algorithm that can effectively sample the search space and identify optimal solutions is necessary to optimise a function. Many algorithms are available for function optimization, but it is crucial to set a baseline to determine the practicable solutions for a given problem. This thesis defines a new objective function (misfit function). The function is proposed for obtaining the elastic parameters of an anisotropic focal region, and these parameters are calculated by using machine learning algorithms such as Grid Search, Random Search, Simulated Annealing, and Nelder-Mead.
Potansiyel alanlarda otomatik değerlendirme teknikleri ve arkeolojik alanlara uygulanması
Bu çalışma, çoğunlukla otomatik veya yan otomatik değerlendirme yöntemleri olarak isimlendirilen jeofizik değerlendirme tekniklerinin (Normalize Edilmiş Tam Gradyan (NTG), Euler Dekonvolüsyon Yöntemi (EDY)) ve bu yöntemlere ek olarak Yatay Gradyantm maksimum noktalarının yerlerinin araştırılması yönteminin (Sınır Analiz Yöntemi (SAY)) arkeolojik alanlarda uygulanmasını içerir. İlk iki yöntem kaynak manyetik yönüne ait ön bilgi gerektirmemektedir. Sonuç olarak, bu yöntemler jeolojik bilginin az olduğu alanlarda başarılı şekilde uygulanabilirler. NTG yönteminde, manyetik veriler için Fourier katsayıları filon yöntemiyle hesaplanmış ve türevlerinden hareket ederek NTG değerleri hesaplanmıştır. Basit modellerin etkisi, NTG değerlerinin aşağı doğru analitik uzanımından elde edilen NTG kesitleri üzerinde araştırılmıştır. Aşağı doğru analitik uzanıma dayanan bu yöntemin temeli, uzanım değerinin doğru hesaplanması değil, tekil noktayla ilişkisini gösteren NTG alan karakteristiğinin elde edilmesi olduğuna değinmek önemlidir. Euler homojenite eşitliği, basit kaynaklar üzerinde manyetik anomalilerin değerlendirilmesi için, yeni bir teknik geliştirilmesinde kullanılmıştır. Euler homojenite ilişkisine dayanan EDY, manyetik verilerin otomatik olarak hızlı değerlendirilmesinde yaygın olarak kullanılır. Derinlik değerinin hesaplama kalitesi çoğunlukla manyetik kaynağın doğasıyla ilgili anomali azalma oranım tanımlayan ve yapısal indeks olarak bilinen parametreye bağlıdır.m Bu yöntemlere ek olarak, yapı sınırlarının belirlenmesi amacıyla, yatay gradyan genliklerinin maksimum noktaların yerlerinin harita düzlemi üzerindeki dağılımından yararlanılmıştır. Yukarıda değinilen bu yöntemler önce kuramsal modeller üzerinde sınanmış, daha sonra da Çorum-Ortaköy Şapinuva Hitit arkeolojik kentinden elde edilen manyetik verilere uygulanmış ve başarılı sonuçlar elde edilmiştir. Anahtar sözcükler : Arkeomanyetik Veri, Normalize Edilmiş Tam Gradyan, Tekil Nokta, Euler Dekonvolüsyon, Yapısal İndeks, Yatay Gradyan, Maksimum Nokta Yerinin Araştırılması
Long period amplification of seismic waves inIstanbul metropolitan area
Istanbul is a megacity located very close to the North Anatolian Fault and thus is highly vulnerable to seismic hazards. During the 2019 Mw5.7 Silivri earthquake the recorded displacements show site amplification at long-periods and prolonged duration corresponding to the frequency range of 0.1-1.0 Hz. The excitations in these frequency range cannot be explained with variations in very shallow structure only and requires an analysis that considers deeper sedimentary basins. Amplifications in this frequency range cause resonance effects and serious damage tall and high-rise buildings and infrastructure. The Mw 5.9 Silivri earthquake of September 26, 2019, provided a good dataset for investigating site-specific amplification effects. This thesis analyzes acceleration records from nine AFAD strong-motion stations distributed over different geological units to evaluate the characteristics of long-period amplification. As an initial test, we compare synthetic waveforms using two velocity: (i) a crustal model from Karabulut et al. (2020), and (ii) an AFAD-based station specific shallow velocity model which includes near-surface velocity layers from AFAD station reports. We show that neither of the models work well for amplified waveforms especially on the motion recorded in stations on the European part of Istanbul. Instead we generate synthetic seismograms for each station. We try two approaches. First, we generate synthetic waveforms at the surface of the crustal model which is assumed to represent a hard-rock reference waveform. Then we randomly generate 2 layer structures on top of this structure and optimize the fit to the data in the frequency domain. As a second approach we calculate the synthetic waveforms at a depth of 2 km. We then again optimize for each station the layered structure that represent first two km beneath the station. For each trial we calculate the amplified waveforms using the transfer function of these two layers for SH waves. For each station we generate 5000 velocity models and optimize the fit between the acceleration spectrum of data and synthetics. The best-fitting velocity structure was obtained by spectral fitting of synthetic and recorded waveforms. Results show that the site amplification largely varies with local geological conditions: stations located on stiff geological units (e.g., 3405, 3413, 3417) have minimal amplification, while soft sediment sites (e.g., 3411, 3412, 3416) undergo strong amplification and long shaking. Station 3412 displays anomalous amplification and longer oscillations probably due to very thick unconsolidated sediments or local site complexities. While the AFAD-based velocity model underestimated the observed amplification, the crustal model fits better for hard rock sites but failed to capture the amplification at soft sediment stations. Based on our modeling results, we estimate that stations 3407, 3411, 3413, and 3416, which are situated on soft sediment, have sediment thicknesses ranging between 120 and 200 meters with underlying layers exhibiting shear wave velocities of 248 m/s, 87 m/s, 420 m/s, 188 m/s for first layer, respectively. For the second layer values for each station 254 m/s, 294 m/s, 789 m/s, 220 m/s for station 3407, 3411, 3413, 3416 respectively and the layer thickness for second layer ranging between 200 m to 1000 m. The better fit at a depth of 2 km for these stations suggests that deeper sedimentary layers significantly influence wave amplification. In comparison, AFAD's velocity models provide shear wave velocities of 597 m/s for 3407, 323 m/s for 3411, 452 m/s for 3413, and 420 m/s for 3416 for first layer and 2000m/s for 3407, 523 m/s for 3411, 772 m/s for 3413 and 849 m/s for 3416 for the second layer, indicating some discrepancies between our estimated values and the reference models. These differences highlight the need for further refinement, considering potential lateral variations and deeper structural influences. When compared to AFAD's reference models, our synthetic seismograms show good agreement for stations 3407, 3411, 3413, 3416 where both amplitude and spectral content are well captured. However, discrepancies at other stations suggest that additional factors, such as lateral heterogeneities or path effects, may be influencing the results. In general, for significantly amplified stations on the Anatolian side, it is to refine these models further, it is necessary to incorporate additional earthquake records from different directions to assess whether the observed variations persist across different source locations and azimuths.
Heyelan duraylılığının jeolojik ve jeofizik yöntemlerle değerlendirilmesi (Gümüşhane Süleymaniye örneği)
Ülkemizde jeolojik, iklimsel, coğrafik özellikleri ve yanlış arazi kullanımı nedeniyle heyelan olayları sıkça yaşanmakta ve çoğu kez tekrarlanarak afete dönüşmektedir. Heyelanlar; akma, kayma, düşme ve devrilme gibi farklı hareket mekanizmalarda gerçekleşmekte olup depremler, ani-şiddetli yağışlar ve sıcaklık değişiklikleri ile tetiklenerek tehlikeli olabilecek afet olaylarına yol açmaktadır. Bu nedenle uzun süreli ve bol yağışın etkili olduğu, nemli iklime sahip Doğu Karadeniz Bölgemizde heyelanların afete dönüşerek büyük ölçüde can ve mal kaybına yol açmasını önleyici mühendislik çalışmalarına ihtiyaç duyulmaktadır. Kayan kütlenin ve altındaki yapının özelliklerinin yüksek doğruluk oranıyla belirlenmesi heyelan yapısının ortaya çıkarılmasında çok önem arz etmektedir. Bu çalışmada, Gümüşhane ili Süleymaniye Kayak tesis alanında jeolojik ve jeofizik yöntemler kullanılıp heyelan duraylılık analizleri yapılarak tehlike oluşturabilecek heyelan alanlarının ortaya çıkarılması amaçlanmıştır. Bu kapsamda, jeofizik yöntemlerle heyelanın sınırları, kayma yüzeyinin derinliği, heyelan kütlesinin farklı kayaç kesimleri, yeraltı suyu durumu, temel kayanın derinlik, yapı ve bileşim özellikleri tespit edilerek sonlu elemanlar ve limit denge analizleri ile düşey ve yanal deformasyonlar ortaya koyulacaktır. Bu değerlendirmeler sonucunda riskli alanlar tespit edilip en uygun iyileştirme yöntemleri önerilecektir.